In wire-cut EDM, electrical discharge machining may be performed with the workpiece submerged in a processing tank that is filled with a machining fluid. A wire electrode runs between an upper guide head and a lower guide head with the two guide heads mounted on an upper arm unit and a lower arm unit, respectively.
In such machines, an opening is made in the processing tank for inserting the lower arm unit with the lower guide head into the tank. Positioning of the workpiece relative to the wire electrode is accomplished by moving the processing tank in two orthogonal directions in a plane parallel to its base. One direction is parallel to the axis of the lower arm unit and the other direction is parallel to the tank wall having the opening.
In this type of wire-cut EDM apparatus, the space between the opening and the lower arm unit must be sealed to prevent leakage of the machining fluid from the processing tank. Various configurations for sealing proposed in the past are illustrated in U.S. Pat. No. 4,565,915 to Girardin, U.S. Pat. No. 4,918,279 to Babel et al., U.S. Pat. No. 4,647,747 to Goto, U.S. Pat. No. 4,808,786 to Shinkai, et al., and U.S. Pat. No. 4,992,640 to Tanaka, et al. As shown in FIG. 21, some of these conventional arrangements use a sealing plate 4 provided between the opening 3 and the lower arm unit 1. Sealing members are then placed between the sealing plate 4 and the wall of the processing tank 2 and also between the wall and the lower arm unit 1. The positioning of sealing members between the sealing plate 4 and the lower arm unit 1, however, creates a problem in terms of frictional resistance, also called the sealing resistance. Among other problems, the sealing resistance may cause the lower arm unit to deform thereby degrading machining accuracy.